Cascadia Subduction Zone Marine Geohazards

Science Center Objects

Societal Issue: Uncertainty related to rupture extent, slip distribution, and recurrence of past subduction megathrust earthquakes in the Pacific Northwest (northern CA, OR, WA, and southern BC) leads to ambiguity in earthquake and tsunami hazard assessments and hinders our ability to prepare for future events.

A map shows relief of a long stretch of continental margin with features on land and in the ocean labeled with lines and words.

Figure 1. Topo-bathymetric map of the Cascadia subduction zone. Cascadia megathrust fault (white line); approximate shelf break along 200-m isobath (yellow line); MTJ, Mendocino triple junction.

Overview

The Pacific Northwest is home to the Cascadia Subduction Zone, a 1,300 km-long tectonic boundary between the Juan de Fuca and Gorda oceanic plates to the west and the North America continental plate to the east (Fig. 1). The denser oceanic plates are converging with North America and subducting beneath the western edge of the continent. The interface between upper and lower plates is defined by a large fault, called a megathrust, as well as numerous smaller faults that cut through the surrounding crust (Fig. 2). Over time, frictional stresses accumulate along these faults, building slowly until they exceed the fault’s strength, resulting in an earthquake. Subduction zone earthquakes are the largest on Earth, reaching magnitudes in excess of magnitude M9, and are known to generate large tsunamis.

A cross-section illustration that shows the features of a subduction zone where oceanic and continental plates collide.

Figure 2. Schematic cross-section of the accretionary wedge along the Cascadia subduction zone. Modified from Moore and others, 2007.

As part of a broader collaborative effort within the USGS (Gomberg and others, 2017) and academia (for example, SZ4D) to advance subduction zone science and improve hazard assessment, we are working with our partners to integrate multi-resolution geophysical imaging and geological sampling to characterize offshore margin morphology, including submarine landslides, sediment delivery systems, seafloor seeps, active faults, and upper plate velocity structure in Cascadia. Because uplift and subsidence associated with megathrust earthquakes often crosses the coastline, coordinated onshore-offshore synthesis is vital to this effort.

Globally observed correlations between marine forearc morphology and structure and megathrust earthquake slip, magnitude, and rupture length along subduction margins suggest possible correlations between seismic slip behavior and morphotectonic variability. Systematic morpho-tectonic analyses in Cascadia highlight distinct along-strike variations in morphology and structure that may reflect spatial variations in megathrust earthquake behavior (Watt and Brothers, in review). Identification and regional mapping of active fault structures will enable further investigation of the links between megathrust behavior and upper plate morpho-tectonics. We plan to investigate the relationships between offshore geologic structure, seafloor morphology, seafloor seeps, and sediment dispersal pathways and depocenters. These efforts will help to identify potential tsunamigenic faults, offshore areas prone to strong shaking, portions of the slope susceptible to landslides, and/or optimal environment(s) for marine paleoseismology studies. Together, this information will provide information fundamental to improving regional hazard assessments and our ability to prepare for future events.

Key scientific questions

  • To what extent and how frequently do potentially tsunamigenic upper plate structures rupture with the megathrust?
  • How do along strike variations in the morphology and structure of the overriding plate relate to possible segmentation of the megathrust?
  • How do the material properties of the wedge vary along the margin, and does this influence the behavior of the underlying megathrust fault? 
  • How is sediment delivered and redistributed across the continental shelf and slope, and how does that influence the geologic record of past events?
  • How does earthquake shaking translate to slope failure, and which areas may be prone to failure in the future?
  • What is the role of fluids in subduction zone processes and related hazards?
  • How is strain partitioned offshore within the overriding plate?

 Main Goals

  • Acquire state-of-the-art high-resolution bathymetric, seismic-reflection, ocean bottom seismometer (OBS), and sediment core data across the offshore Cascadia margin
  • Apply advanced geophysical and core processing techniques and integrative analyses to identify and characterize submarine active faults, landslides, and sedimentation patterns that may present potential hazards to coastal areas
  • Work with scientists on land to link processes and hazards across the shoreline

Targeted Products

  • Earthquake recurrence history
  • 3D fault/geologic model
  • Quaternary sediment distribution and properties
  • Bathymetry/backscatter and seep distribution
  • Hazard maps: earthquake, landslide, and tsunami
  • Event response, such as rapid OBS deployments

Fieldwork Timeline

A table illustration to show a timeline for fieldwork by date and topic.

Map illustration of the seafloor off the continental coastline shows all of the regions and tracklines of recent fieldwork.

Figure 3. Index map of U.S. Cascadia margin showing where data have been collected since 2018 as part of the Subduction Zone Marine Geohazards Project. Details of each survey effort are provided. See the list under the heading, "Fieldwork Completed."

Fieldwork Completed

Since 2018, the USGS and its partners have completed several research cruises spanning the entire U.S. Cascadia margin (Fig. 3).

Exciting things to come in 2021!

Margin-wide Cascadia seismic reflection imaging and ocean bottom sensor deployments [See also, the informational sheet]

Partners/Collaborators

Federal: NOAA, BOEM, NSF, EXPRESS, U.S. National Park Service, and scientists from multiple USGS Mission Areas (Natural Hazards, Ecosystems, Core Science Systems, Energy and Minerals)

State: California Geological Survey, Washington Department of Natural Resources (DNR)

Academic: University of Washington, Humboldt State University, Oregon State University, Scripps Institution of Oceanography, University of Hawaii, Woods Hole Oceanographic Institution

Private: MBARI, Ocean Exploration Trust

Related USGS Projects

John Wesley Powell Center for Analysis and Synthesis Working Groups

  1. Margin-wide geological and geophysical synthesis to understand the recurrence and hazards of great subduction zone earthquakes in Cascadia
  2. Tsunami Source Standardization for Hazards Mitigation in the United States

News

Deciphering Cascadia’s history of mega-earthquakes using MBARI’s unique deep-sea vehicles
- MBARI’s 2019 online annual report